The manufacture of cast poly(methyl methacrylate) is an important industrial process. Casting of methyl methacrylate monomer containing some polymer, commonly referred to as syrup, had been carried out in cells prepared from plate glass, flexible gaskets and steel clips (see e.g., Encyclopedia of Polymer Science and Technology, Volume 1, pp. 276-279; Interscience Publishers, 1964). Since this type of process is slow, taking 16-170 hours, continuous processes have been developed, see e.g., U.S. Pat. Nos. 3,872,197 and 4,046,850. While the residence time in these continuous casting units is much shorter than in the cell casting operation, the expensive nature of these units make it desirable to increase the production rate as much as possible.
Many initiators have been disclosed for use in increasing the production rate of cast poly(methyl methacrylate) sheet. For cell casting, the following initiators are but a few examples; benzoyl peroxide and t-butyl hydroperoxide combinations in U.S. Pat. No. 2,632,758; a number of peroxide and azo initiators, with benzoyl peroxide and 2,2'-azobis (isobutyronitrile) being preferred in U.S. Pat. No. 3,639,553. For continuously cast sheet, U.S. Pat. No. 3,872,197 recites a large number of catalysts (column 7, lines 30-40), but only uses azobisbutyronitrile and azobisvaleronitrile in the examples. Similarly U.S. Pat. No. 4,046,850 recites a large number of initiators (column 4, lines 10-31) but only azobisisobutyronitrile, azobisdimethylvaleronitrile and lauryl peroxide are used in the Examples. The use of an initiator such as t-alkyl peroxyneodecanoate insures fast polymerization; but this initiator gives sheet with relatively high residual monomer. The less active azobisisobutyronitrile gives too low polymerization rates.
The use of other initiators, such as azobisdimethylvaleronitrile in large quantities allows fast production rates, but the cast sheet contains a number of voids, believed to be caused by nitrogen generated from the azo initiators. With other initiators, the rate of heat generation from polymerization is such that voids may be caused by the excessive temperature of the heat of polymerization. Attempts have been made to overcome these problems by use of peroxycarbonate initiators, as described in U.S. Pat. No. 4,211,742; however, some low molecular weight peroxycarbonates require extremely careful handling because of their rather unstable nature and the potentially violent explosion and fire accompanying their decomposition if allowed to warm even to room temperature. On the other hand, safer, higher molecular weight peroxycarbonates are more expensive on a molar basis; thus, will cost more to use in sheet casting.
In an attempt to overcome the above mentioned disadvantages of previously disclosed initiator systems, D. W. Wood, Plastics Engineering, May 1975, pp. 51-53, recommends the combined use of t-butyl peroxyneodecanoate and decanoyl peroxide. However, the last paragraph of this article states, "The resulting cured castings were hard and glasslike, and exhibited some cavities that could be attributed to shrinkage during polymerization of the material." An acrylic sheet containing cavities is undesirable. When Wood's experiments were repeated by applicant, the void formations were observed, and the unconverted monomer in the polymers was 8.9 and 11.4%. Good quality sheet should have less than 2% residual monomer.
It is desirable to have an initiator that will provide a fast production rate while at the same time permit production of sheet free of defects and relatively free of residual monomer.